Speleothems (a.k.a. the
tropical ice cores)
Cave deposits

Laminated structure of CaCO3
 Evaporation-deposition
 Slow CO2 degassing == minimal
disequilibrium with drip waters
 Rapid CO2 degassing ==
disequilibrium precipitation
 Possibility for water-soil or
water-rock chemical
equilibration == undermines
reliability of speleothems as
paleothermometers
Schwartz (2007)
Geochemistry of speleothems



T (°C) = 16.5 - 4.3 (d18Oct - d18Ow) + 0.14 (d18Oct - d18Ow)2 (Epstein et al.
1953)
d18Oct would also reflect precipitation patterns
 Caveat: the “amount” effect in d18Ow (increase in d18O during heavy rain
events)
Age control:
 14C - have to correct for nonradioactive C introduced into DIC from
dissolution of calcite in the soil zone
 U/Th - have to account for detrital 230Th in ‘dirty’ calcite
Geochemistry of speleothems

Have to estimate d18O of drip water:
 Close to annual average of d18Oppt
 But local alteration due to surface evaporation, transpiration,
selective recharge of seasonal precipitation (snow meltwater)
 Some assumptions and ways to estimate dw:


Secular variation in d18Oppt = annual (seasonal) variation (assumes
a particular rate of change with time)
Analysis of fluid inclusions in the speleothems (dD of the water
should be unchanged)


dDppt = 8 d18Oppt + d0 (d0 is deuterium excess, 10permil)
Using models for variation in d18Oppt (derivation of water vapor from
seawater with known variability in d18O)
Paleoclimate studies

Matching changes
in North Atlantic
climate and South
China
 Greenland cooling;
coincidentally,
enhanced
summer
monsoon rains
in South China,
reflected in the
isotopes
Wang et al. 2001
But what about the carbon?


Fractionation between DIC and calcite is very small; so, d13C
reflecting isotopic composition of drip water
d13C in calcite affected by:
 C3/C4 plants balance; C4 - higher d13C
 Plant vegetation density above cave -- dissolution by reaction w/
atm = higher d13C
d13C increases with
decreasing rainfall
due to decrease in
the contribution of
HCO3- to the drip
water (Frumkin et
al. 1999,2000)